In recent years, a digital device system in which plural home digital devices are interconnected is on its way to allowing a user to view various kinds of contents with high-quality image and sound. For example, using HDMI (High-definition Multimedia Interface) designed in December 2002, the digital device system can output various kinds of contents with high-quality image and sound. The HDMI standard defining specifications of such HDMI is an interface standard that allows video, audio, and control signals to be transmitted via a single cable, and is an input-output interface standard designed for next-generation AV (Audio-Visual) devices that input and output digital video and digital audio.
The HDMI standard defines the CEC (Consumer Electronics Control) protocol as a protocol to transmit a control signal bidirectionally. By using the CEC protocol, the control signal is relayed via plural AV devices. This allows the user to control the plural AV devices with a single remote control. To be more specific, since the control signal is bidirectionally transmitted between, for example, a television set (referred to as the “TV” hereafter), an AV amplifier, and a DVD player-recorder (referred to as the “DVD recorder” hereafter), the user can operate an entire home theater configured by the TV, the AV amplifier, and the DVD recorder with the single remote control.
HDMI has not only a characteristic that no signal deterioration is caused because of the digital transmission, but also a characteristic that a reproduction device such as a DVD recorder can read display identification data of a destination device and thus adjust its own output formats of audio and video data in order for the destination device to reproduce the data. Moreover, as another characteristic, HDMI can encrypt the audio and video data and transmit the encrypted audio and video data, so as to prevent illegal copying.
In HDMI, a DDC (Display Data Channel) bus is used for unidirectionally transmitting the display identification data from a device, such as a TV or an AV amplifier, that has an HDMI input port (referred to as the “HDMI input device” hereafter) to a destination device having an HDMI output port (referred to as the “HDMI output device” hereafter). Here, the “DDC” refers to a standard defined by VESA (Video Electronics Standards Association), as a function of informing the destination device of model information and the like of a display in order to implement a “plug and play” display.
The display model information and information of supported audio and video formats is stored as EDID (Enhanced Extended Display Identification Data) in a memory of the HDMI input device. The EDID is data that is read, via the DDC bus, from the HDMI input device connected to the HDMI output device, basically in response to a change (or a pulse) from Low to High of an HPD (Hot Plug Detect) signal caused by the HDMI input device. More specifically, when the HPD signal is High, this means that the HDMI output device can read the EDID of the HDMI input device. The audio and video formats described in the EDID suggest that the TV and the AV amplifier serving as the HDMI input devices have the capability to reproduce the digital audio and video data in the described formats (note that the HPD signal becomes Low when the destination device is turned off or is not connected).
Here, when the device for outputting video is different from the device for outputting audio, audio and video that are supposed to be in synchronization with each other may be out of synchronization. In other words, a so-called lip-sync error may be caused because the video output lags behind the audio output. To address this problem, the HDMI Standard Version 1.3a (see Non-Patent Reference 1) discloses an audio-video data synchronization method whereby the EDID describes each latency in the audio and video output processing (referred to simply as “the latency” hereafter) so that the output device can automatically adjust the audio output and the video output not to cause a lip-sync error. According to the method described in Non-Patent Reference 1, when the latency changes in the HDMI input device, the HDMI input device updates the information regarding the latency described in the EDID. Then, the HDMI output device reads the changed EDID and automatically adjusts the audio output and the video output. Here, when the HDMI input device updates the EDID, the HPD signal needs to be Low. Moreover, when the HDMI output device reads the EDID, the HPD signal needs to be High. For these reasons, according to the method described in Non-Patent Reference 1, when the latency is updated, a change (or a pulse) from Low to High occurs to the HPD signal.
Moreover, the HDMI standard adopts HDCP (High-bandwidth Digital Content Protection System) as an authentication protocol for verifying whether or not the destination device is valid when content that requires copyright protection is to be outputted. It is defined that authentication processing is started in response to a change (or a pulse) in the HPD signal from Low to High.
That is to say, in the case where the lip-sync error is corrected according to the audio-video data synchronization method described in Non-Patent Reference 1, the HDMI output device performs unnecessary authentication processing whenever the latency is updated. Thus, there is a disadvantage of causing a problem of “blackout” described later.
The aforementioned conventional audio-video synchronization method is explained with reference to the drawings, as follows.
FIG. 1 is a diagram showing an example of a configuration of a conventional HDMI-connected audio-video output system. As shown in FIG. 1, a DVD recorder 100, an AV amplifier 200, and a TV 300 are HDMI-connected.
The DVD recorder 100 includes a single HDMI output port 100PO. The AV amplifier 200 includes a single HDMI input port 200PI and a single HDMI output port 200PO. The TV 300 includes a single HDMI input port 300PI.
The DVD recorder 100, the AV amplifier 200, and the TV 300 are connected via HDMI cables. The audio data and the video data are transmitted unidirectionally via the HDMI output port 100PO, the HDMI input port 200PI, the HDMI output port 200PO, and the HDMI input port 300PI in this order. Moreover, a control message is transmitted bidirectionally over a control signal bus on HDMI. Here, note that the present system is in a state where video is outputted from the TV 300 and audio is outputted from the AV amplifier 200 (referred to as the “theater mode” hereafter) (on the other hand, a state where both audio and video are outputted from the TV is referred to as the “TV mode”).
FIG. 2 is a diagram for explaining operations of the devices in the conventional HDMI-connected audio-video output system. The horizontal axis in the diagram indicates the passage of time (the same applies to FIGS. 3, 11, 12, and 17 described later).
In the case where the maximum resolution of the video format supported by the TV 300 is 1080 p, the EDID shown in (f) describes information, as an initial value, indicating that 1080 p is supported and also describes a period of time taken before the video is outputted to a display device, namely, video latency. As the video latency, “100 ms” is set in the case of 1080 p. The detailed explanation of other descriptions in the EDID is omitted. It should be noted that since the TV 300 and the AV amplifier 200 are validly HDMI-connected, the HPD signal outputted from the TV 300 to the AV amplifier 200 is set to High as shown in (e).
In the theater mode, the AV amplifier 200 outputs the audio. In doing so, as shown in (f), the AV amplifier 200 adjusts (delays) the audio output according to the video latency obtained from the EDID read from the TV 300. Thus, the video latency is not set in the EDID of the AV amplifier 200 shown in (c) (or, set to “0 ms”). It should be noted that since the AV amplifier 200 and the DVD recorder 100 are validly HDMI-connected, the HPD signal outputted from the AV amplifier 200 to the DVD recorder 100 is set to High as shown in (b).
The DVD recorder 100 reads the EDID of the AV amplifier 200 that is shown in (c), and sends the audio and video data.
FIG. 3 is a diagram for explaining operations performed in the conventional HDMI-connected audio-video output system when an output format for the video data outputted from the DVD recorder 100 is changed.
The video latency caused in the case of standard-definition 480 p is smaller than the video latency caused in the case of high-definition 1080 p. Here, suppose that the format for the video data outputted from the DVD recorder 100 is changed from 1080 p to 480 p at a time T11. When the AV amplifier 200 adjusts timing of the audio output according to the video latency in the case of 1080 p, a synchronization error is caused between the audio output from the AV amplifier 200 and the video output from the TV 300 (that is, the audio output lags behind the video output). To address this, for example, when the TV 300 detects that the video latency is changed from 100 ms to 30 ms while receiving 480 p data and starting the video output after the time T11, the TV 300 temporarily changes the HPD signal to Low as shown in (e) (at a time T12). Then, while the HPD signal shown in (e) is Low, the TV 300 updates the video latency described in the EDID shown in (f) to, for example, 30 ms. After this, the TV 300 changes the HPD signal shown in (e) to High again (at a time T13).
When detecting that the HPD signal of the TV 300 shown in (e) is changed from Low to High (at the time T13), the AV amplifier 200 reads the EDID of the TV 300 shown in (f). Then, the AV amplifier 200 changes the HPD signal shown in (b) from Low to High (at a time T14).
When detecting that the HPD signal of the AV amplifier 200 shown in (b) is changed from Low to High (at the time T14), the DVD recorder 100 determines that a change may be occurring to the HDMI connection status. In order to adhere to the HDCP, the DVD recorder 100 starts the authentication as to whether or not the destination device is valid. When starting the authentication, the DVD recorder 100 stops outputting the audio and video data that requires content protection. Then, the DVD recorder 100 reads the EDID of the AV amplifier 200 shown in (c) again, and validates the AV amplifier 200 while transmitting the video data (such as blackout) that does not require content protection. After validating the AV amplifier 200, the DVD recorder 100 resumes outputting the audio and video data (at a time T15). The HDCP authentication is not a principal objective of the present invention, and therefore a detailed explanation thereof is omitted.
As described thus far, according to the audio-video data synchronization method described in Non-Patent Reference 1, whenever the video latency changes in the device for outputting the video, the EDID of the video output device is always rewritten. Then, the audio-video reproduction device stops outputting the audio and video data (i.e., the content) that requires content protection, and performs the HDCP authentication. As a result, the audio-video data synchronization method described in Non-Patent Reference 1 has a problem of causing a few seconds during which no content is outputted before the audio-video reproduction device completes the authentication and resumes outputting the audio and video data.
On the other hand, a method that does not employ the EDID has been proposed to address a lip-sync error caused between the outputs of the audio and video data (see Patent Reference 1, for example). Patent Reference 1 discloses a technique whereby information about the video latency is provided using a control message from a video output device (TV) to an audio output device (AV amplifier).    Non-Patent Reference 1: High-Definition Multimedia Interface Specification Version 1.3a    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2006-33436